1. Field of the Invention
The present invention relates to a method for manufacturing a plastic molding, and particularly to a method for manufacturing molded product having a a predetermined shape, in which a piece of material containing a foaming agent is molten and plasticized by heating through the application of high frequency voltage, so that the foaming agent dissolves.
2. Description of the Related Art
A plastic molding having an inner foamed construction which can be, for example, used as a decorative and protective elongated molding attached to a side panel or door panel of an automobile is light in weight and has a high shock-absorbing or damping effect, in comparison with a solid molding having a rigid inner construction.
Such a foamed plastic molding can be manufactured in a heating process in which a raw resin material containing a foaming agent is introduced in molding dies provided with high frequency electrodes and is heated and plasticized by the application of a high frequency voltage, so that the foaming agent dissolves to form a plastic molded product having a predetermined shape.
In the process mentioned above, the product obtained has the largest cell construction at its center core portion, since the inner foaming agent dissolves by the application of the high frequency power. The cell becomes small toward the outer periphery thereof, and the outer surface of the product forms a dense and hard shell. This kind of separate construction consisting of a foamed core and a hard shell is advantageous, particularly when it is used to make a decorative and protective molding for an automobile.
However, the conventional manufacturing process mentioned above has a drawback that gas which is issued from the raw resin material when foaming takes place and an air which exists in a molding cavity defined by the molding dies tend to remain in the cavity without being completely discharged therefrom, thus resulting in the production of so-called air marks or undesirable small depressions on the outer surface of the product. This results in the bad appearance of the product. Particularly, an elongated molded product, such as a decortive and protective molding for an automobile often has the disadvantage of an ugly appearance, since passages for which the gas and the air to escape from are limited.